The invention relates to a push-pull saturation converter with two switching transistors alternately charging the primary circuit of a transformer with a DC voltage at the input of the push-pull saturation converter. The control inputs of these switching transistors is linked via a common base winding of the primary circuit at a voltage divider.
In switching converters, the conversion of one DC voltage into one or more electrically separated DC voltages is undertaken. Accordingly, the input DC voltage is reshaped into a rectangular voltage in the kilohertz range by means of fast switching transistors and is transmitted with the assistance of a transformer. The transformer serves for the electrical separation of the output and input circuits and, in Blocking Oscillator type converters, also serves for energy storage. In order to obtain the required output DC voltage, the secondary voltage of the transformer is recitified and filtered.
Essentially, three circuit designs are known for switching converters, these being employed depending upon the power range or the properties specifically required, namely single-ended converters, push-pull converters and push-pull saturation converters.
Single-ended converters are subdivided into single-ended flow converters and single-ended blocking converters whereby, given the single-ended flow converter, energy is transmitted via a diode into a choke in the load circuit during the conductive phase of the switching transistor and the choke then emits the stored energy via a free-running diode into the load circuit during the blocking phase of the switching transistor. In contrast thereto, given a single-ended blocking converter, energy is stored in the transformer during the conductive phase of the switching transistor and the transformer emits the stored energy into the load circuit via a diode in the blocking phase of the switching transistor.
The primary disadvantage of single-ended converters is the difficulty, given changes of load, of keeping a plurality of output voltages coupled out within a prescribed output voltage range. Moreover, a high expense for components is required and an efficiency of less than 80% is achieved in standard circuits. Since the short angle of current flow of said circuits effects high internal resistances of the output circuits, additional regulators must often be employed in the output circuits, such regulators signifying an additional expense and a further reduction of the efficiency.
In externally controlled push-pull converters, two switching transistors are switched in succession, shifted by half the period duration. When the first switching transistor is switched on, then the second blocks and current flows via a diode into a choke in the load circuit. When the second switching transistor is now switched on, a current flow then derives via a second diode and the choke in the load circuit. Due to this manner of operation, the transformer is magnetized in both directions. This means that the charges of induction are twice as high as in the single-ended flow and single-ended blocking converters, whereby a significantly increased output power derives. The symmetrical drive of the two switching transistors, however, requires suitable control circuits with a considerable circuit expense.
With the third principle, that of push-pull saturation converters, two switching transistors are provided as in the push-pull converters. However, the switching transistors are linked with their bases via a common winding of the primary circuit and are driven via said winding which, for example, is linked via a voltage divider to the input voltage of the push-pull converter. Thereby, its function is such that the transistors with higher gain is first through-connected and then remains conductive via the base current impressed by means of the base winding until the transformer becomes saturated and the collector current of the transistor which is quickly increasing due to the drop of the inductance becomes so high that the base control current can no longer hold the transistor in saturation and the voltage at the transformer reverses due to the decreasing magnetic induction.
By so doing, the base voltage at the transistor with higher gain simultaneously becomes negative and the second transistor becomes conductive until, after renewed saturation of the transformer, the operation effects another reversal.
Push-pull saturation converters have an angle of current flow of nearly 360.degree. and, thus low internal resistances. The attainable output power corresponds to those of externally controlled push-pull converters since the transformer also becomes magnetized in both directions here and a great change of induction thereby derives. Given constant input voltage, one obtains output voltages which are largely constant in a broad load range. If a plurality of output voltages are required, then these can be taken without further expense with rectifiers in accordance with the secondary windings. Circuit expense for the drive of the transistors is eliminated.
Despite these advantages and the favorable, low-cost realization, the use area of the saturation converter is limited to the transmission of small powers with a low degree of efficiency. The cause of these limitations are the problems in the switching operation of the power transistors. Since the transformer is almost suddenly placed in saturation, the collector current in the current-conducting transistor increases very quickly. As a result of a base current excess, said transistor is in the saturation state at this time and, since it continues to be supplied with base current, a relatively long storage time passes until the shutdown is initiated due to the increase of the collector-emitter voltage. A disproportionately high collector peak current arises which cannot only dangerously overload the transistors but, above all results in high switching losses. Moreover, the high peak currents effect strong voltage fluctuations at the input and output of the converter as well as potent, high-frequency disruptions which make the elimination of radio interference difficult. The energy content of the current peak is intermediately stored in the leakage inductance of the transformer and is subsequently fed back into the input circuit on the one hand, which effects dangerously high voltage peaks at the shutoff transistor, and, on the other hand is fed to the output circuits, whereby output voltages undesirably increase, particularly given small load. Since the current peaks of both transistors are usually of different height, a great asymmetry is caused in the transformer, this effecting an unequal load distribution to the output diodes and switching transistors and causing undesired voltage waviness or ripple in the outputs.